Parkinson's Disease (PD) is a slowly developing syndrome. The syndrome evoked by MPTP, in contrast, has been observed almost exclusively as a sudden eruption evoked by relatively high acute doses. An animal model of PD capable of evaluating the therapeutic efficacy of neural transplants should embody the progressive features of idiopathic PD. It should also make provision for precise measures of function that can be relied on to trace the development of the syndrome and its response to this therapeutic strategy. To achieve such a model, monkeys will be treated chronically with low doses of MPTP calculated to induce a progressive impairment of motor function. Throughout the course of an experiment, function will be evaluated by a variety of measures. These measures include behavioral analyses, neurochemical analyses and morphological analyses. The data from this study should be of significant benefit in assessing the potential clinical role of nonfetal donor tissues in treating Parkinson's disease.